The present invention relates to a method for measuring anchoring strength of liquid crystal and to a measurement system therefor.
Literature in relation to this technical field includes the following.
(1) Hiroshi Yokoyama, xe2x80x9cPrinciple and Practical Method for Measuring Interface Anchoring Strength: Focused on Strong Electric-Field Method,xe2x80x9d Liquid Crystal, 4, 63 (2000).
(2) Hiroshi Yokoyama, xe2x80x9cThermodynamics of Interface Anchoring of Liquid Crystal,xe2x80x9d Liquid Crystal, 3, 25 (1999).
(3) H. Yokoyama: Mol. Cryst. Liq. Cryst., 165, 265 (1988).
(4) H. Yokoyama and H. A. van Sprang: J. Appl. Phys., 57, 4520 (1985).
Anchoring at a substrate-liquid crystal interface is determined by three anchoring energy coefficients defined by the following expression (1):                     Δγ        =                                            1              2                        ⁢                          W              p                        ⁢                          δθ              2                                +                                    1              2                        ⁢                          W              a                        ⁢                          δφ              2                                +                                    W              pa                        ⁢            δθδφ                                              (        1        )            
where xcex8 is a zenith angle that a director forms with respect to the substrate normal line (z axis), xcfx86 is an azimuth angle, and xcex3 is interface free energy (see the above-mentioned literature (1) and (2)).
Over the years, there have been developed methods for measuring anchoring energy, which affects the photoelectric response of liquid crystal and which is the most fundamental physical property of orientation film (see the above-mentioned literature (3) and (4)).
For measurement of strength of zenith angle anchoring, a strong electric-field method utilizing the scaling law of Frank elasticity has been developed. This method enables highly accurate extraction of characteristics of interface anchoring without being affected by bulk characteristics such as elastic constant and dielectric constant.
The strong electric-field method is used for nematic liquid crystal having a positive dielectric anisotropy. In this method, a vertical electric field is applied to a cell; and an optical retardation R that remains in the vicinity of the substrate interface at that time is measured as a function of voltage in a high voltage region where voltage is greater than the Freedericksz transition voltage.
The retardation R is the total sum of contributions from individual points; i.e., is represented by the following expression:                     R        =                                            2              ⁢              π                        λ                    ⁢                                    ∫              0              ∞                        ⁢                          Δ              ⁢                              xe2x80x83                            ⁢                                                n                  eff                                ⁡                                  (                  θ                  )                                            ⁢                              xe2x80x83                            ⁢                              ⅆ                z                                                                        (        2        )            
where xcex is the wavelength of light, and xcex94neff(xcex8) is effective birefringence of light at the respective point.
According to the theory of elasticity, in general, there holds the scaling law in which not only retardation, but also other summable physical property values change in proportion to the reciprocal of the density of electric flux applied to a cell.
Moreover, in bulk, liquid crystal is oriented along the direction of an electric field, and no retardation is generated for a light ray passing through the liquid crystal in parallel to the electric field (Rxe2x86x92 for Vxe2x86x92∞). The strong electric-field method is based on these two characteristics.
Meanwhile, for azimuth angle anchoring, no summable physical property which automatically vanishes at the limit of a strong electric field is known. Therefore, the strong electric-field method cannot be applied to azimuth angle anchoring as is. Therefore, at present, there are used only several easier methods in which influences of bulk are removed through comparison with simulation results.
Further, in the case of zenith angle anchoring as well, there exist various factors which hinder Rxe2x86x920, such as discrepancy between a light ray and an electric field and a residual retardation of the substrate; and for strong anchoring their influences cannot be ignored.
An object of the present invention is to provide a method for measuring anchoring strength of liquid crystal through measurement of magnetic-field-induced torque, which, unlike conventional methods such as the strong electric-field method, uses a strong magnetic field, is therefore based on a clear principle, does not require bias correction, and enables measurement for liquid crystal of an arbitrary shape, as well as a measurement system for the measuring method.
To achieve the above object, the present invention provides the following.
[1] A method for measuring anchoring strength of liquid crystal, characterized by applying a strong magnetic field to a liquid crystal cell, and measuring magnetic-field-induced torque to thereby measure the anchoring strength of liquid crystal.
[2] The method for measuring anchoring strength of liquid crystal as described above in [1], wherein the strong magnetic field is about 10 T.
[3] A system for measuring anchoring strength of liquid crystal, characterized by comprising an optical system for radiating light onto a liquid crystal cell; means for applying a strong magnetic field to the liquid crystal cell; and means for measuring magnetic-field-induced torque.
[4] The system for measuring anchoring strength of liquid crystal as described above in [3], further comprising means for adjusting the angle of the liquid crystal cell.